Explicit directive simulation for method arguments and avoid implicit conversion

explicit_method.cpp

#include <iostream>
#include <string>
#include <vector>
#include <type_traits>

struct A
{
 operator const char*() {
     return m_str.c_str();
 }
 std::string m_str;
};


template<typename T>
void f(const T *str) {
    static_assert(std::is_same<T, char>::value, "str must be char*");
}


int main()
{
  A a;
  const char *pa = nullptr;
  f(a);  // Fails
  f(pa);  // Works
}